Microstructure evolution and properties of NiTiCrNbTax refractory high-entropy alloy coatings with variable Ta content

Abstract

In this work, the novel NiTiCrNbTax (x = 0.1, 0.3, 0.5, 1) refractory high-entropy alloy (RHEA) coatings were successfully synthesized on Ti6Al4V by laser cladding technology. The effect of Ta content on microstructure, thermal stability, micro-hardness and corrosion resistance of RHEA coatings was investigated in detail. The results showed that NiTiCrNbTax RHEA coatings were composed of BCC, NiTi2-Laves and Cr2Nb-Laves phases. With Ta content increasing, the formation of BCC phase was promoted while the precipitate of Cr2Nb-Laves phase was affected. The average micro-hardness values of Ta0.1, Ta0.3, Ta0.5 and Ta1.0 RHEA coatings were calculated as ~922.8 HV0.3, ~985.1 HV0.3, ~851.3 HV0.3 and ~825.4 HV0.3, respectively. The decrease of micro-hardness value might be ascribed to the reduction of hard Laves phase. After annealing at 900 °C for 3 h, the phase compositions of all RHEA coatings remained unchanged but the grains of Cr2Nb-Laves phase were refined obviously. Besides, RHEA coatings exhibited excellent corrosion resistance and the minimum corrosion current density was 1.08 × 10−7 A/cm2. Moreover, the secondary passivation phenomenon emerged in Ta0.5 and Ta1.0 coatings, and the passive film mainly consisted of the oxidation state of Ti, Cr, Nb, Ta and the sub-oxide state of Nb, Ta.